It is possible Guttenberg heard about the Korean printing press from traders? Yes. However, given the distances involved at that time, there is almost no chance he got to examine a working model.
I think you’re charitable. That such a piece of information, from more than half a world away, would make it to a person willing and able to act on it without the reference making its way into any other contemporaneous western sources, is more than just a bit of a stretch